GraphQL is a query language for APIs and a runtime for fulfilling those queries with your existing data. Developed internally by Facebook in 2012 and publicly released in 2015, it provides a more efficient, powerful, and flexible alternative to traditional REST APIs.

Unlike REST, which requires loading from multiple URLs, GraphQL APIs get all the data your app needs in a single request. You describe your data using a type system, and clients can ask for exactly what they need, nothing more and nothing less. This makes apps fast and stable, even on slow network connections.

Key concepts include:

• Queries: Requests for reading or fetching data.
• Mutations: Operations for writing, modifying, or deleting data.
• Schema: The core component that defines the types and relationships in your API.

For developers building a website, this means you can fetch multiple, nested resources in one call, reducing the number of requests to your hosting server. This improves performance and can simplify your frontend code. It's particularly useful for complex applications and systems where different clients (web, mobile) need different data views.

In summary, GraphQL is a declarative data-fetching specification that puts the client in control, leading to more efficient data loading and a better developer experience for modern web and mobile applications.